This invention relates to a detector for obtaining two-dimensional radiation image using a scintillator and a phosphor as radiation detecting medium. The detector is characterized in that by combining a scintaillator and phosphor of short fluorescence life with wavelength shifting fibers, two-dimensional radiation image can be detected at high speed and with good positional precision even if the incident radiation has high flux. If combined with a neutron converter, the detector is also capable of two-dimensional neutron imaging and hence is used for studies in materials physics and structural biology by neutron scattering in nuclear reactors and accelerators or X-ray scattering with synchrotron radiation, for medical X-ray diagnosis using X-ray generator and accelerator, and for autoradiography using X-rays or neutrons. The detector is also used in understanding dynamic events through fast processed and real-time radiation image detection with a radiation image detector for studies in high-energy physics using an accelerator, as well as in a function apparatus for monitoring the distribution of radiations including neutrons generated in nuclear reactors and fusion reactors.
The two-dimensional radiation image detector has been used to determine the position of entrance of a radiation into a scintillator or a phosphor by detecting the emitted fluorescence with bundles of optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a horizontal and a vertical direction. In the case of a detector using scintillators, one pixel need be formed by one scintillator as shown in FIG. 31 [K. Kuroda et al., Nucl. Instr. and Meth. A430 (1999) 311-320] or FIG. 32 (Masaki Katagiri, JPA 2000-187077), so scintillators of a comparatively large size have been used to construct a two-dimensional radiation image detector that has a positional resolution of at least about 5 mm and which has a comparatively large area.
In the case of phosphors, a thin fluorescent sheet is used and detection is realized by bundles of optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a horizontal and a vertical direction, so in order to enhance the detection efficiency of fluorescence, the distance between adjacent bundles of optical fibers has to be shortened; hence, the use of phosphors has been limited to a two-dimensional radiation image detector that has a positional resolution of no more than about 2 mm and which has a comparatively small area.
In either type of detector, scintillators or phosphors having short fluorescence life are used and detection is realized by using optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a horizontal and a vertical direction, so except for some phosphors such as ZnS:Ag that emit a large amount of fluorescence, no more than several tens of photons reach the photodetector. Therefore, utilizing the emission of fluorescence that occurs upon incidence of radiation according to the Poisson distribution, K. Kuroda et al. have proposed a method by which the fluorescence from horizontal and vertical wavelength shifting fibers is converted to a two-dimensional image by a signal processing system comprising a photomultiplier tube, an amplifier circuit for amplifying the output signal, a peak height discriminating circuit for determining signal timing, a circuit for generating pulses of a specified time duration, and a coincidence circuit [K. Kuroda et al., Nucl. Instr. and Meth. A430(1999) 311-320]. In this method, in order to assure the desired minimum efficiency of simultaneous detection, the coincidence time which is set to perform simultaneous counting of signals corresponding to the horizontal and vertical directions has been chosen at specified values at least twice as much as the fluorescence life.